This sounds like a no-brainer, but it's worth understanding. Windows keeps a record of the applications you use most often and reorders them on your hard drive to optimize their launch. This record is continually updated in the file Layout.ini; the actual file moving takes place every few days, while the computer is idle.

In addition to moving files, Windows tracks the programs and processes initiated immediately after your system boots up, creating a list of files to prefetch during boot-up (meaning that Windows anticipates your next move and grabs necessary files before the computer even needs them). Windows performs a similar task each time you use an application, because when you launch an app, you're not just running the executable file; a number of files and program components load. In theory, these actions will make Windows run faster the more you use it.